Variable resistor



Jan. 4, 1944. R, SCHADE 2,338,458

[VARIABLE RESISTOR Filed June 10, 1940 2 Shets-Sheet 1 Jan. 4, 1944. R,scH 2,338,458

VARIABLE RESISTOR Filed June .10, 1940 2 Sheets-Sheet 2 a; J J P0000Samoa 4-, I mrvzqy.

Patented Jan. 4, 1944 VARIABLE RESISTOR Rudolf Schade, Finkenkrug,Osthavelland, Germany; vested in the Alien Property CustodianApplication June 10, 1940, Serial No. 339,725 In Germany June 10, 1939 9Claims.

The invention relates to variable resistors having a contact cooperatingwith the resistor and which instead of the switch devices hitherto knownare to be employed alone or in connection with residual current circuitbreakers to interrupt power circuits. Such resistors carry a very highload and must be varied very rapidly, so that disturbing influencesoccur at the contact surface which may so unduly stress the resistor andcontact as to damage them within a short time. These influences are onthe one hand the cause of unduly high current densities with subsequentexcessive heating and wear and tear, particularly at one edge of thecontact, and on the other hand of a sparking at this edge with similarconsequences. According to the invention such influences are avoided bysuitable means.

The invention is illustrated in the accompanying drawings, in which-Fig. 1 is a side elevational view of a typical form of a resistor,illustrating schematically, the nonconformity of current pathscharacteristic of such devices in prior use;

Fig. 2 is similar to Fig. 1, except that the current paths are of asmaller load than shown in Fig.

Fig. 3 is a similar elevational view showing a form of resistorcontemplated by the invention in which the specific resistance of thematerials of the resistor are varied, and the change in the distributionof the current paths efiected thereby is illustrated diagrammatically;

Fig. 4 is a side elevational view shematically showing a differentembodiment of the invention in which different specific resistivity ofparts is obtained by constructional features other than thenon-homogeneousness of the bodies comprising the resistor;

Figs. 5 and 6 are modifications of the resistor shown in Fig. 4;

Fig. 7 is a perspective view of a modified resistor in which specificresistivity is obtained by the shape of the resistor; and

Figs. 8 to 12 are side elevational views of a schematic nature, showingmodifications of the resistor illustrated in Fig. 7.

The current is caused according to the invention to flow along suchpaths in the resistor, contact or in both parts as to prevent an unduecrowding of the current paths at one edge of the corresponding contactsurface. Fig. 1 of the drawings illustrates the undesired crowding ofthe current paths if the contact s consists as is usual entirely ofmetal. If the conductivity of the contact is indefinitely great ascompared to the resister w, the current paths are crowded at the edge kof the contact s spaced from the current junction u even if they enterthe resistor at the current junction a uniformly distributed. Thecurrent density is also indefinitely great at the current junction. Thisis due to the unequal length of the current paths along the upper andlower side of the resistor. The detrimental effects mentioned above arealready brought about upon merely approaching the theoretical limitillustrated in the drawings, that is to say, in the case of the definiteconductivity of the usual metal contacts.

According to the invention the crowding of the current paths isprevented either by suitably selecting or distributing the specificresistance of the resistor, contact or of both parts, also by giving theone or the other part a suitable shape, further by suitably distributingthe contact resistance over the corresponding contact surface as well asby suitably distributing the contact pressure over this surface or byselecting and dis tributing the conductive resistance. The means.whereby this may be accomplished, may also be combined in various ways.

The resistance in the different current paths may be made equal to acertain extent, if the contact and resistor are made of the samematerial. The distribution of the current paths thereby obtained isroughly illustrated in Fig. 2. In this case the contact edge It carriesa considerably smaller load than according to Fig. 1, this, however,being disadvantageous for certain instances. The resistance in thecurrent path 0 extending along the upper surface of the resistor w andthe left-hand surface of the contact 8 is smaller than that in thecurrent path 11 extending near the lower surface of the resistor w andnear the right-hand surface of the contact s. A greater current densitywill therefore be brought about in the neighborhood of the left-handcontact edge than in the neighborhood of the edge Z. Consequently, theresistance in the different current paths between the two currentjunctions or terminals aand b of the resistor to and contact srespectively is rendered according to the invention as uniform aspossible.

An effective improvement is obtained if the specific resistance of thecontact varies along the surface in engagement with the resistor. Also avariation of the specific resistance of the resistor contributes inbringing about the same resistance in the different current paths. Thestipled surface illustrated in Fig. 3 indicates according to theinvention the variations of the specific resistance of the resistor wand contact s, the points at which the dots are more crowded indicatinga greater resistance. Along the contact surface the specific resistanceof the resistor 20 is greatest. The current flowing from the currentjunction a tends therefore to follow paths towards the lower side of theresistor, i. e., away from the dangerous point It. By increasing thespecific resistance of the contact 8 with respect to its edge is thecurrent paths may also be shifted away from this edge so as to preventan undue current density at this edge.

The specific resistance of the contact and that of the resistor may alsovary according to the same law along the surface serving for the passageof the current, so that the drop of the specific resistance is approx.the same in both bodies.

Fig. 4 shows an embodiment of the invention which is somewhat lesscomplicated than the non-homogeneous bodies to and 8 according to Fig.3. These bodies consist as shown in Fig. 4 of individual layers l 4 ofdifferent specific resistance, the different shadings indicating adistribution of the specific resistance and of the current path asillustrated in Fig. 3.

The change in resistance is also adapted to the manner in which thecontact moves. If the contact slides on the resistor it may bepreferable to uniformly vary in opposite directions the specificresistance of both parts along the surface serving for the passage ofthe current. Fig. 5 shows such an embodiment in which the contact isdesigned as is also the resistor in the form of a rod or plate. 1

Both bodies consist of sections 1 5 of a different specific resistance,arranged in opposite sequence. As will be seen from the current pathsshown in Fig. 5 the distribution of current at the contact surface issymmetrical with respect to the center of the latter. Consequently, anundue current density cannot occur at this edge. The main portion of thecurrent passes between parts of the same specific resistance (here, forinstance, between 2-2). It would even sufiice when giving the resistorand contact the same shape to make both parts of approx. the samematerial having the same specific resistance, since also the symmetry ofthe distribution of current is then ensured. The form of the inventionshown in Fig. 5 is, however, more advantageous, since the specificresistance at the outer edges of the two parts sliding on one another isgreatest and an undesired increase in current density is avoided atthese points. The two parts to and s may be moved in opposite directionsin order to increase the control speed.

If the resistor and contact or one of these bodies is given acylindrical shape, i. e., the one body rolls on the other, the specificresistance along the rolling surface serving for the passage of thecurrent may vary preferably in the same direction along this surface.The rolling body consists, for instance, as shown in Fig. 6 of sectors l5 of different specific resistance. Both the resistor and the contactare preferably subdivided into sections of difierent resistances. Inthis case the arcs of the sectors of the rolling body should be equal tothe lengths of the sections of the resistor, so that only parts of equalspecific resistance come into contact. Consequently, the specificresistance in the contact increases in the same direction as that in theresistor.

Figs. '7 to 12 show further instances for preventing an undue crowdingof the current paths or at least their causes by giving the bodiesemployed a suitable shape.

The shape may be given either by varying the cross-section or thelongitudinal section of the resistors or contacts.

According to Fig. 7 the cross-section of the resistor and contact isenlarged at the outer side of the angle or are formed by the currentpaths. The apex of said angle coincides with the edge of the contact.About this edge the current paths are bent in an angleor arc-like manneras will be seen, for instance, from Figs. 2 to 4. Since the innermostcurrent path with respect to the edge It is the shortest, the width ofthe resistor and contact is smallest at this point according to Fig. 7and is enlarged towards the opposite crosssection edge The center ofgravity of the current carrying cross-section is thereby shifted fromthe dangerous point and the heat produced is therefore more uniformlydistributed. Furthermore, the point k of the greatest current density isbest cooled.

In the following embodiments the resistors, contacts or both are givensuch a shape in the longitudinal section that the current paths have thesame length. According to Fig. 8 the longitudinal section of theresistor to and according to Fig. 9 of the contact s is so curved thatthe center of curvature is opposite to the current junction of the otherpart. In this manner the current path lying near the edge 7c islengthened and a concentration of the current in this path is prevented.According to Fig. 10 the longitudinal section of the contact has awedge-like shape, whereby the current path in said section extendingnear the edge is is longer than that extending near the edge I. Alsothis contributes to bring about a uniform distribution of current in thecontact cross-section. A similar shape may also be given to the resistorw. In order to attain the same length of the current paths the surfacesa and b of the resistor w and contact 8 may be inclined with respect tothe central current path in these bodies and, if desired, curved in asuitable manner, as will be seen from Fig. 11. A similar effect may beattained according to Fig. 12 by the fact that the current paths at theinner side of the angle or are formed by the same are lengthened, forinstance, by notches 6 provided in the contact s. All the embodimentsdescribed above may be combined in any suitable manner.

If the distribution of current in the contact surface is to be rendereduniform by the selec tion of the contact resistance, the contactresistance at the edge k of the contact near the current junction a ofthe resistor must be made greater than that at the opposite edge Z. Thecontact resistance must decrease uniformly or gradually towards the edgeI in order that the current paths be shifted away from the dangerousedge is of the contact. The contact resistance may be influenced bysuitably machining the brush surface in contact with the resistor, bymetal or insulating material coatings suitably distributed over thissurface or in any other suitable manner.

The distribution of current over the contact surface may be furtherinfluenced by gradually increasing the contact pressure on this surface.The contact pressure is preferably smallest at the edge is of thecontact 8 and increases towards the opposite edge 1. This may beefiected in a simple manner by an asymmetrical current load of thecontact according to the invention. The contact depending upon themanner in which it is secured to its support may be subjected to atilting moment which tends to tilt it about the edge Z.

The invention herein disclosed has been set forth with particularity asto certain forms or embodiments for illustrative purposes, but it isobvious to those skilled in the art that modifications may be madewithout departing from the scope of the invention as defined by theappended claims.

What is claimed is:

1. A variable resistor for controlling heavy currents, comprising a bodyof resistance material and a contact member also formed of resistancematerial, said member having a surface in sliding engagement with saidbody at points spaced lengthwise thereof, and said body and contactmember being composed of sections of different specific resistivity,said sections being arranged with respect to their resistivity so thatthe current paths have substantially the said resistance, whereby thecurrent is caused to flow through the body and contact member alongpaths which do not converge at the edge of the contact surface which isnearest to the terminal end of the said body.

2. A variable resistor for controlling heavy currents, comprising a bodyof resistance material and a contact member also composed of resistancematerial, said contact member having a surface in sliding engagementwith said body which is at least approximately equal in area to thetransverse cross-sectional area of said body, and said body and contactmember being composed of sec-, tions of different specific resistivity,said sections being so arranged with respect to their resistivity thatthe current is caused to flow through the said body and contact memberalong paths which are substantially parallel in said body and which havesubstantially equal resistances.

3. A variable resistor comprising a resistance body and a contact memberin slidable engagement therewith, said resistance body and contactmember having portions of diiferent specific resistivity, whereby'undesirable concentration of the current paths through the resistancebody and contact member is prevented by a gradation in the specificresistivity of different portions of the said body and contact such thatthe shorter current paths extend through material of higher specificresistivity.

4. In a variable resistor, a block of resistance material having aterminal at one end, a contact member having one end in slidingengagement with said block and having a terminal at the other end, thesaid block and contact member being composed of parts of differentspecific resistivity, and the said parts being so distributed withrespect to the length of the current paths which extend through theblock and contact member from terminal to terminal that said currentpaths are of substantially the same resistance.

5. A variable resistor for controlling heavy currents, comprising twoblocks of resistance material disposed substantialy at right angles toeach other with the end of one block in sliding engagement with the sideof the other block, terminals for said blocks at the ends thereof, eachterminal establishing a conductive contact with the associated blockthroughout the entire area of the end thereof, and current paths ofequal resistance connectin all points on the contact face of oneterminal with corresponding points on the contact face of the otherterminal.

6. In a variable resistor for controlling heavy currents, two bodiesformed of resistance material and disposed in sliding engagement witheach other, terminals for including said bodies in an electricalcircuit, and current paths extending through said bodies between saidterminals which are made equal in resistance by gradation in thespecific resitivity of different parts of said bodies.

7. In a variable resistor for controlling heavy currents, a resistancebody built up of layers differing in specific resistivity, a terminal atone end of said body in direct conductive connection with all saidlayers, a contact block also built up of layers differing in specificresistivity, said block having a contact surface intersecting all thelayers thereof in sliding engagement with that layer of said body whichhas the highest specific resistivity, and the said block being sooriented that the layer thereof having the highest specific resistivityis nearest to the terminal end of said body, and a terminal for saidcontact block in direct conductive connection with all the layersthereof.

8. A variable resistor for controlling heavy currents, comprising arigid self-supporting slab of resistance material having a metallicterminal at one end thereof, a contactor comprising a block ofresistance material having opposite ends of substantially the same area,one end having a metallic terminal secured thereto and the other endbeing in sliding engagement throughout its area with the side of saidslab, and current paths extending from terminal to terminal through saidslab and contactor which are uniformly distributed over the area ofcontact between the slab and contactor, said extension and distributionof current paths being effected by providing said slab and contactcrwith portions having different specific resistivity and arranging theportions so that the shorter current paths extend through the resistancematerial of higher specific resistivity.

9. In a variable resistor, a block of resistance material and a contactmember in slidable en gagement therewith, terminals for including saidblock and contact member in an electrical ci cuit, and current pathsextending through said block and contact member which are uniformlydistributed over the area of contact between the block and contactmember by a gradation in the specific resistivity of different partsthereof.

RUDOLF SCI-IADE.

